In thermal ink-jet printing, droplets of ink are selectably ejected from a plurality of drop ejectors in a printhead. The ejectors are operated in accordance with digital instructions to create a desired image on a print sheet moving past the printhead. The printhead may move back and forth relative to the sheet in a typewriter fashion, or the linear array may be of a size extending across the entire width of a sheet, to place the image on a sheet in a single pass.
The ejectors typically comprise capillary channels, or other ink passageways, which are connected to one or more common ink supply manifolds. Ink is retained within each channel until, in response to an appropriate digital signal, the ink in the channel is rapidly heated by a heating element disposed on a surface within the channel. This rapid vaporization of the ink adjacent the channel creates a bubble which causes a quantity of liquid ink to be ejected through an opening associated with the channel to the print sheet. The process of rapid vaporization creating a bubble is generally known as "nucleation." One patent showing the general configuration of a typical ink-jet printhead is U.S. Pat. No. 4,774,530, assigned to the assignee in the present application.
In order to create nucleation of a vapor bubble in a quantity of liquid ink, the liquid ink in a specific small area adjacent to the heating element must be brought to a sufficiently high temperature. Although a heating element exposed to the liquid may be provided with a generally sufficient "burn voltage," not all of the area of the heating element exposed to the liquid ink will be of a sufficiently high temperature to create nucleation. It has been found that the central portion of the effective area of a heating element reaches a sufficiently high burn voltage well before areas around the periphery of the heating element area. That is, the heating element may have a sufficiently high temperature only in a relatively small central area thereof, while the outer-lying portions of the heating element area will not be of a sufficient high temperature, and therefore nucleation will occur only for the liquid ink disposed toward the center of the heating element area. This phenomenon causes any number of practical drawbacks, such as: unpredictability of the volume of a resulting ejected droplet; a wasting of energy when a significant portion of the heating element is continually brought close to, but not over, the sufficient temperature; and an undesirable accumulation of excess heat within the printhead.
The phenomenon of the center of a heating element area consistently being hotter than the outer portions of the heating element area can in large part be traced to a lateral leakage of heat energy from the heating element area to other areas within the printhead. By "lateral" is meant that a significant component of heat energy is lost along the edges of the perimeter formed by the area of the heating element exposed to the liquid ink. In the case of a "side shooter" design of a printhead, where the heating elements are typically formed on a main surface of a silicon chip, this lateral heat leakage can be significant.